High speed dissemination of failure information in mesh networks

ABSTRACT

A method for high speed dissemination of information about faults in spans that interconnect nodes in a telecommunication mesh-type network, the method comprises creating a Failure Updating channel (FU channel) in the network and circulating Failure updating messages (FU messages) via the FU channel between the network nodes, wherein each of the FU messages issued by a particular node is disseminated to its adjacent node(s) and carries information about all spans in the network currently available to the particular node, by presenting status indications of the spans in respective information slots of the FU message.

FIELD OF THE INVENTION

The present invention relates to a technique for distributinginformation about failed links between nodes of a mesh network

BACKGROUND OF THE INVENTION

Mesh networks, contrary to ring networks, are those comprising a numberof inter-linked star-like connections between NE (network elements). Aconnection between two adjacent NE is usually called a span.

To perform urgent path restorations and other reconfiguring actions inthe network when failures occur, either NEs or any managing entityshould be informed on the current condition of each span in the network,and in the minimal time.

Presently, this service action is usually provided by a so-callednetwork management station (NMS) of the network, via service channels.The NMS has a network level view of the “network health status”. NMSreceives notifications and alarms about failed links and re-calculatesnew trails in the network to bypass such links. Theinformation/instructions with respect to the new corrected trails isthen downloaded to the relevant network elements. The described processis known as a restoration process. In mesh networks, it takes aconsiderable time for execution due to complexity of configuration ofsuch networks.

The OSPF protocol (Open Short Path First protocol) which is widely usedin IP networks relates to the family of “link state protocols”. Thisprotocol enables exchanging the routing information which has beenupdated due to the network topology changes (span addition/deletion,failure). It is a complex and highly software dependent protocol. Aconsiderable portion of the time is required for updating NMS and eachand every node in the network about the new status of the “networkhealth”. Furthermore, the restoration cannot be accomplished if the NMSis down by any reason.

OBJECT OF THE INVENTION

It is therefore the purpose of the present invention to provide atechnique overcoming the above-mentioned drawbacks, i.e., enablingaccelerated dissemination of failure information in mesh networks.

SUMMARY OF THE INVENTION

The solution proposed below allows for the high speed dissemination ofthe information on faults in a mesh network, enabling for fastrestoration of the faulty paths. For non-mesh networks, the restorationtime up to 50 ms is stated by a number of standards of internationalstandard bodies such as ITU-T, ANSI, BELLCORE. This time is counted forcompleting the restoration after a fault is detected and, by now, hasbeen considered too short for mesh networks due to their complexity. Theproposed solution enables reaching this limit in the mesh networks, too.

To ensure high speed dissemination of information about faults in spansthat interconnect nodes in an optical mesh network, there is provided amethod which comprises creating a Failure Updating channel (FU channel)in the network and circulating Failure Updating messages (FU messages)via said FU channel between the network nodes, wherein each of the FUmessages reflects status of all spans in the network.

In practice, each FU message may comprise a number of information slotsnot less than the number of spans in the network, wherein each of theslots being adapted to carry a status indication of a particular networkspan in a form of a predetermined code.

Preferably, the FU channel can be created in the information channel ofthe network. For example, the FU channel can be ensured using overheadbytes of a SONET/SDH frame or multiframe for periodically transmittingsaid FU messages via said bytes.

Alternatively, when the invention is employed in an optical transmissionsystem such as OTN, a so-called OSC channel can be used as the FUchannel. The OSC channel is an Optical Supervisory Channel transmittedin parallel with the information optical channels between networkelements.

Let an input FU message be a FU message entering a network node from anadjacent node during a time period t (in a simplified example, can becalled clock t), and carrying status information on all spans of thenetwork, which information is available to the adjacent node. Thenetwork node may receive one or more input FU messages during apredetermined time period.

Let an output FU message be a FU message issued by the network nodeafter the time period t (for example, at clock t+1) and comprisingupdated status information on all spans of the network, which becameavailable to the network during the time period t.

Let a previous output FU message be an FU message issued by the networknode during the time period t (or at clock t), and comprising theprevious status information on all network spans known to the nodebefore the updating (i.e., collected by the time period t, or at clockt−1).

According to the method, each network node permanently (per clock)receives one or more input FU messages from its adjacent nodes andissues its output FU message to the adjacent nodes.

To understand the above method, the following definitions are to beintroduced. Any particular node in the network is considered to be

-   -   an originating node (ON) with respect to at least one associated        span (ONS) on which the node is able to directly detect one or        more events (such as alarms) and based on that originates status        of the ONS and disseminates it in the network via its output FU        message, and    -   a non-originating node (NON) with respect to any other network        span (NONS) not associated with said node, the non-originating        node does not originate status of any NONS but judges about a        status of a particular NONS based on status information        concerning this NONS, being available at said node owing to the        FU messages circulation; each of the nodes therefore operating        in two modes being an originating mode and a non-originating        mode.

In the method, the step of circulating the FU messages (actuallycomprising circulation of status information on all the network spansbetween all the network nodes) implies performing the following actionsat each network node and at each clock:

-   -   for one or more spans (ONS) associated with said node (ON),        originating respective status information;    -   for spans (NONS) not associated with said node (NON), forming        respective status information based on the previous status        information and status information in one or more input FU        messages available in said node;    -   combining the status information originated for ONS and the        status information formed for NONS, and forwarding it to all        adjacent nodes as the output FU message of said node.

In view of the above, to perform the step of originating statusinformation for the ONS spans, the method comprises steps of:

-   -   defining a state machine (SM) of an originating node, assigned        to a particular span (ONS) associated with its originating node        (ON), the state machine comprising more than two states and        providing transitions from state to state; said states and        transitions depending on presence or absence of said one or more        alarms or other events on a particular span (ONS), wherein the        states of the state machine (SM) respectively correspond to        possible statuses of a particular span (ONS);    -   providing at each node of the network one or more said state        machines (SM) respectively assigned to one or more spans (ONS)        associated with said node as with their originating node,

thereby ensuring that status of each particular span of the network isoriginated and permanently updated at its associated ON.

To ensure forming the status information for the NONS spans (by thenodes in the non-originating mode —NON), the method comprises:

-   -   storing, at each node of the network, a previous output FU        message of the node reflecting the previous status information        on all spans of the network known to the node;    -   applying, at each node of the network, status rules for forming        status information for the NONS spans based on the respective        previous status information and respective status information in        the one or more input FU messages.

Based on the above definitions and explanations, the process ofcirculating the FU messages in the mesh network is performed in such amanner that:

-   -   each node receives a number of input FU messages from its        adjacent nodes and issues its output FU message to the adjacent        nodes;    -   each node acting in the originating mode with respect to its        particular associated span (ONS) originates status thereof in        the output FU message based on the previous status of said span        and the alarms/events if detected on said span, (according to        the corresponding state machine SM and regardless a status of        said span in any of the FU messages received at the node),    -   each node acting in the non-originating mode with respect to        span(s) (NONS) not associated to it, updates the previously        known status of any particular NONS in the output FU message,        using the status rules.

It should be noted that the process of circulating is performedindependently at any node i.e., adjacent nodes of a particular node maysend to it different FU messages during a stabilization period of the FUmessages, and after that stabilization period their FU messages shouldbecome the same. The stabilization period is checked and statedempirically—the larger the network, the higher the period.

The state of stability is achieved as a result of gradually (step bystep) updating status of spans at each network node based on the inputFU messages and alarms the node receives, and by continuously (step bystep) disseminating the updated status of each span by issuing an outputFU message from the node.

Preferably in the above method, the status rules used for updating thestatus of a particular span NONS in the output FU message of a nodecomprise a table of priorities. These rules and tables are logicallyidentical at all nodes throughout the network.

In the table of priorities, the priorities are arranged by taking intoaccount:

-   -   the previous status of a particular NONS (transmitted by said        node in the previous output FU message),    -   status(es) of the particular span NONS received at the node in        one or more FU messages,    -   direction of transitions of the state machine SM.

According to one version proposed by the Inventors in the frame of themethod, the priorities are stated as follows:

the highest priority is assigned to a status of the particular span NONScorresponding to such a state of the state machine SM which ispositioned one transition forward from the state of said machinecorresponding to the previous status of the NONS span,

the priority lower than but next to the highest is assigned to theprevious status of said NONS span, i.e. to its unchanged status,

the lowest priority is assigned to a status of the particular NONS spancorresponding to such a state of the state machine SM, which ispositioned one transition backward from the state of the state machineSM corresponding to the previous status of the NONS span.

The above defined rules and priorities can be applied when NON (i.e.,the node in a NON mode) receives one or more FU messages during apredetermined time period (or at a clock).

Preferably, the step of updating status of NONS at NON further comprisesthe following steps:

-   -   if the status of a particular NONS span is equal in all input FU        messages which can be received at the node, accepting said equal        status for the output FU message regardless a previous status of        said NONS (and therefore the priority of said status);    -   when receiving input FU messages carrying different statuses        with respect to one and the same NONS, selecting for the output        FU message such a status of said span which has the highest        priority among those received at said NON including the previous        status of said NONS.

According to another aspect of the invention, there is proposed a systemfor implementing the described method in mesh networks, the systemcomprising a fault updating (FU) channel for circulating input andoutput FU messages between nodes of the network, the FU messagescarrying status information on all network spans; the FU channel beingarranged by providing each node in the mesh network with means forreceiving the input FU messages from adjacent nodes and for transmittingthe output FU messages to the adjacent nodes; each node being providedwith a memory and a processing means for updating status information ofall network spans in the output FU messages it issues.

In the system, each of the nodes is preferably provided with means fordetecting events such as alarms on one or more network spans associatedwith said node and a state machine SM for originating status informationconcerning said spans (originating spans ONS) based on said alarms; andalso provided with status rules for updating status informationconcerning network spans not associated with said node (non-originatingspans NONS).

The memory of a node being preferably utilized for storing a previousoutput FU message, the events, the SM, the input FU messages and thestatus rules; the processing means being operative to perform the SMtransitions to obtain status information of the ONS spans, capable ofapplying the status rules to form status information of the NONS spans,and adapted to combine the obtained status information of all networkspans in an output FU message of the node.

Details of the invention will become apparent as the descriptionproceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described and illustrated with the aid ofthe following non-limiting drawings in which:

FIG. 1 schematically illustrates a mesh network.

FIG. 2 schematically illustrates an FU (Fault Updating) message.

FIG. 3 illustrates a simplified version of a state machine SM of anoriginating node, used by a node in the originating mode.

FIG. 4 schematically illustrates a simplified table of priorities whichcan be used by a network node as status rules for updating status ofnon-originating network spans (NONS).

FIG. 5 illustrates operation of a network node according to theinvention, using a schematic block-diagram of the node.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a schematic mesh network where nodes N1-N10 areinterconnected by links S1-S14. In this network, node N1 will be theoriginating node (ON) with respect to the associated link S1, sinceaccording to the definition, the node N1 is authorized (or capable) todetect alarms directly from this span and therefore to create and inserta status of this particular link in an FU message which the node N1creates at the time. Simultaneously, the node N1 is a non-originatingnode (NON) for any other link in the network of FIG. 1, since none ofthe links except for S1 can be directly inspected by this node from thepoint of alarms. Owing to that, node N1 may only forward the status ofthe particular link received in an FU message from other node or nodes(in this case, only from node N2).

Likewise, node N2 might be considered the originating node (ON) forlinks S1, S2, S3, while for any other link it can be only anon-originating node NON.

At this point of the description, it should be clarified that only onenode can be the originating node for a particular connection, andtherefore each link is considered to comprise a couple of componentunidirectional spans. For example, the drawing shows that link S1 iscomprised of a span S1 a and a span S1 b. Span S1 a is directed from N2to N1, and N1 is considered its originating node since it is capable ofdirectly sensing alarms from the incoming span. Span S1 b goes from N1to N2, so N2 is the originating node of S1 b.

FIG. 2 schematically illustrates an FU message as a binary map 10 havingas many fields 12, as the number of spans in the mesh network shown inFIG. 1. Each of the fields 12 serves for reflecting a current status ofa specific span, in one-to-one correspondence. Each span may have anumber of statuses, those can be indicated by predetermined binarycodes. In a particular case, each field 12 comprises several bits whichcan be used for carrying a specific binary code. The status of a span isdefined by the presence/absence of one or more specific alarms which aredetected on this span by its originating node, the status is thentransmitted within the FU message to other nodes of the network.

FIG. 3 illustrates a simplified example of a State Machine (SM) of anoriginating node which, according to the invention, is implemented ineach node of the network for each of its associated spans. The SMreflects, by its states, possible statuses of a particular spanassociated with the originating node. This exemplary SM (marked 20)comprises the minimal number of states (3 states) which are as follows:

a. Normal or IDLE (marked 21)—the state when the particular span is inservice with no anomalies. A node functioning in its originating mode(the originating node ON) will introduce the status “idle” in the FUmessage with respect to this particular span. The status will be codedby a predetermined binary code in a corresponding field of the FUmessage.b. Signal Failure (SF), marked 23—when the originating node ON receivesalarms which represent an out-of-service condition (e.g., LOS from itsparticular associated span, it is considered as a signal failure signaland it causes a transition 22 from the state IDLE to the state SF. Thenew state SF of the state machine SM (indicating the status of thisparticular span) is introduced in the FU message produced by theoriginating node, in the form of another binary code in the same field.c. Transient Failure (TF), marked 25—the state which is considered tostart after the SF state is cleared, i.e. when alarms disappear on thespan (the transition 24 is caused by a condition “idle” on the span).The state TF terminates after the clear condition is stable for apredetermined time. During this defined period, the status of the spanin the FU message issued from the Originating Node will be stated as TF.After terminating the TF state the state machine transits to the IDLEstate 21 (the transition 26 occurs when the span condition “idle” lastsfor the predetermined time TE).Direction of transitions in the SM is strictly determined: the statemachine 20 performs transitions from state to state only in thedirection of a→b→c→a.

FIG. 4 illustrates an example of a priority table 28, which can beutilized as rules for making decisions on status of a particular span ata node which is a non-originating node (NON) for this span. (Such a nodeacts in a non-originating mode with respect to this span).

As has been mentioned in the preceding portion of the description,status of a particular span is actively controlled (originated) at itsoriginating node (ON) to which this span is physically related andassigned. For spans not related to a particular node the situation isdifferent: this node acts for them as a non-originating node (NON) whichdoes not actively create (does not originate) their status.

Since, during the network operation, any network node is informed aboutthe current status of many non-related to it spans in the network byadjacent nodes (and these nodes may be updated or not yet updated inrespect of these spans), some behavior instructions are to be used bysuch a node. In the frame of the present application, such instructionsare called status rules of the non-originating nodes. These instructionsmust assist each NON in making correct decisions as of which status of aparticular NONS (non originating span) should be transmitted from theNON in any specific case.

Generally, the rules are as follows:

-   -   a) in most cases, the status of a particular span NONS which was        transmitted by the NON in its previous output FU message (a        so-called previous status) is considered for making decisions on        a new current status of the span; NON makes a decision, whether        to update the previously known status of any particular NONS in        the output FU message, only when the status of said NONS in at        least one FU message received at the node differs from the        previous status of said particular NONS known to said node.    -   b) if all FU messages which can be received at NON from other        nodes comprise equal statuses of a particular span, the new        status of this span for transmitting it from the NON should be        selected equal to that received in the FU messages and        regardless the previous status of the span.    -   c) if a discrepancy is found between the previous status of a        span and its status in a single FU message coming to NON, or        between statuses of one and the same span in two or more FU        messages coming to NON, the discrepancy should be resolved to        the favor of the status information which is considered        “fresher”, among all the considered statuses including the        previous status, from the point of the above-mentioned state        machine SM.

Example 1

let the previous status of a span transmitted from NON be SF, and nowtwo or more FU messages are received at NON, all stating status IDLE forthis span. The NON should issue a new FU message with the status IDLE,since most probably, this node happened to be the last node in thenetwork which is informed on the change of status of this particularspan with a delay.

Example 2

let several different FU messages be received at NON, and there is atleast one FU message where the status of a particular span is one stepdownstream (forward) from the previous status of this span according tothe transition direction of the state machine SM. Let the previousstatus of the span be IDLE and the downstream status be SF. In thissituation, the NON should issue FU message with the SF, i.e., thedownstream status regarding the previous one. The meaning of such adecision—the downstream (forward) status is most probably a “fresher”new update, which is not yet known by other nodes.

The same decision will be made if NON with the IDLE status of aparticular span receives just a single FU message with the SF status ofthis span. The NON node will “believe” to this message and change thespan status to SF.

Example 3

NON receives some FU messages carrying, for a particular span, codes ofthe same (unchanged previous) status and of an upstream status withrespect to the previous status: say, the previous status is IDLE and theupstream (backward) status is TF. In this case, the NON should issue anew FU message with the unchanged status IDLE. The meaning of such adecision—the upstream status is most probably a non-updated version,while our node has likely been updated before the present FU message andalready has a “fresher” status.

The same decision will be made if a single FU message is received: NONwith the IDLE status of a particular span receiving a single FU messagewith the TF status of this span will “not believe it” and leave the IDLEstatus for the span.

The last rule (c) is actually cast into a table of priorities similar tothat shown in FIG. 4, which table is especially useful in case there area lot of specific alarms to be considered (i.e., the State Machine iscomplex), and in case when different FU messages are received at oneNON.

In the table of FIG. 4, the left-hand column shows the previous statusof a particular span (NONS) which is currently known to a specific NON.All the remaining columns are assigned for various statuses of the spanwhich may be indicated in new FU messages incoming this specific NON.Numbers in the boxes of the table indicate priorities, according towhich the NON should make decisions with respect to a new status of thespan. The lower the number, the higher the priority.

The new status selection is provided as follows. Let the NON keeps for aspecific span the previous status TF (see the bottom line).

If it receives three input FU messages, each stating different statusesfor the same span (IDLE, SF and TF), the highest priority status will beselected—it is the status IDLE.

If SF and TF statuses are received in respective two input FU messages,the TF status will be selected since it has the higher priority.

If only one FU message with the SF status is received (but more FUmessages may arrive to this NON), the previous status TF will bepreserved since it has the higher priority than SF.

And, of course, if all the incoming FU messages state one and the samestatus for a particular span (say, all SF) it will be adopted regardlessthe previous status of the span.

In case a node is connected only to one adjacent node and thus can beinformed only by this single adjacent node via a sole FU message (inFIG. 1, node N1 can be informed only by node N2), the status rules willaccept a primitive form (see rule b). In this example, N1 will alwaysagree with statuses of all non-related spans as received in the sole FUmessage from N2.

The method proposed and described above with the aid of the attacheddrawings enables for the maximally swift updating of fault informationin the mesh network.

It should be kept in mind that the illustrated table is a simplifiedexample and more complex tables (comprising more lines and columns andmore variations of priorities) can be built for other more complex statemachines SM and combinations of alarms.

The complex state machines (SM) considering more alarms comprise notonly more states; the transitions may become not so straight forwardi.e., the SM may transit to more than one states from one particularstate, though only one option is to be implemented. In such cases tablesof priorities are the most preferred tool for making decisions on thenew status of a particular span.

FIG. 5 illustrates a schematic block-diagram of a particular node 30comprising a memory 32 and a processor 34. The node is capable ofoperating in the originating mode (schematically marked by dottedcontour 36) for originating spans ONS, and in the non-originating mode(schematically indicated by a dotted contour 38) for non-originatingspans NONS. Both of these modes are performed independently from oneanother and utilize the memory and the processing resources. The memoryaccommodates a program of State Machine of an originating node, StatusRules for non-originating nodes and stores a previous status informationon the network spans (schematically marked 33) known in the node 30 bythe current time period.

Functions of the node can be explained using a descriptive example ofsynchronizing time clocks t, t+1 and t−1. However, the node may workasynchronously and is preferably software-controlled rather thanhardware-controlled. Also, selecting borders of the time periods(clocks) are rather schematic and can be set in a different manner.

The node 30 is capable of detecting alarms/events taking place on theoriginating spans thereof at clock t (this capability is indicated byarrow 40). The alarms and events on the originating spans are processedin the originating mode 36 of the node, using the State Machine (SM) ofan originating node. The processing result is the updated statusinformation of the originating spans ONS of the node.

The node 30 is also capable of receiving a number of FU messages (marked42) from adjacent nodes. Each FU message carries status informationabout all spans of the network at clock (t). Status information aboutthe NONS spans is processed in the non-originating mode 38 of the nodeusing the Status Rules (SR). The processing result received at thenon-originating mode comprises the updated status information about theNONS spans of the node. For both modes, the processor 34 uses theprevious status information 33 available at clock (t), which is based ondata 40 and 42 obtained at clock t−1.

The combined updated status information covering all spans of thenetwork forms an output FU message 44 of the node, which is issued atclock t+1. This message is transmitted from node 30 to adjacent nodes(not shown), and is also stored in the memory 32 to be used, at the nextclock, as a previous status information 33.

While the invention has been described with reference to a number ofparticular examples, it should be appreciated that many variations ofbuilding the failure updating message might be suggested, other alarmsmay be considered, different state machines might be defined fordetermining states of nodes, as well as the status rules may bemodified. Such and other variations should be considered part of theinvention.

1. A method for dissemination of information about faults in spans thatinterconnect nodes in a telecommunication mesh-type network, the methodcomprises creating a Failure Updating channel (FU channel) in thenetwork and circulating Failure Updating messages (FU messages) via saidFU channel between the network nodes, wherein each of the FU messages isissued by a particular node, disseminated to all its adjacent nodes andcarries information currently available to said particular node aboutall spans in the network, by presenting status indications of the spansin respective information slots of the FU message, wherein said FUmessages comprise the following message types: an input FU message beingan FU message entering a network node from an adjacent node during atime period t, and carrying status information on all spans of thenetwork available to the adjacent node; an output FU message being an FUmessage issued by the network node after the time period t andcomprising updated status information on all spans of the network whichhas become available to the network node during the time period t; and aprevious output FU message being an FU message issued by the networknode during the time period t, and comprising the previous statusinformation on all network spans, known to the node by the time periodt.
 2. The method according to claim 1, wherein a number of theinformation slots in the FU message is not less than a number of spansin the network, and wherein each of the slots is adapted to carry astatus indication of a particular network span in a form of apredetermined binary code.
 3. The method according to claim 1,comprising creating the FU channel in the information channel of thenetwork.
 4. The method according to claim 3, comprising creating the FUchannel using overhead bytes of a SONET/SDH frames for periodicallytransmitting said FU messages via said bytes.
 5. The method according toany claim 1, wherein said mesh-type network is an optical networkprovided with an optical supervisory channel (OSC), the methodcomprising using said OSC channel as the FU channel.
 6. The methodaccording to claim 1, ensuring simultaneous operation of each of thenetwork nodes in two modes being an originating mode and anon-originating mode, in the originating mode, a node is considered anoriginating node (ON) with respect to at least one associated span (ONS)on which the node is capable of directly detecting one or more eventsand thereby of originating the span's status, and in the non-originatingmode, a node is considered a non-originating node (NON) with respect toany other span (NONS) in the network, not associated with thenon-originating node.
 7. The method according to claim 1, wherein in thestep of circulating the FU messages each network node receives, duringone time period t, one or more said input FU messages from its adjacentnodes and issues one said output FU message to the adjacent nodes.
 8. Amethod for dissemination of information about faults in spans thatinterconnect nodes in a telecommunication mesh-type network, the methodcomprises creating a Failure Updating channel (FU channel) in thenetwork and circulating Failure Updating messages (FU messages) via saidFU channel among all the network nodes, so that each of the nodes,whenever issuing an FU message, disseminates said FU message to all itsadjacent nodes, wherein the FU message carries information about allspans in the network currently available to said node by presentingstatus indications of all the spans in respective information slots ofthe FU message, and so that each of the nodes updates the informationcurrently available to it based on one or more FU messages received fromits adjacent nodes; wherein the method ensures simultaneous operation ofeach of the network nodes in two modes being an originating mode and anon-originating mode, in the originating mode, a node is considered anoriginating node (ON) with respect to at least one associated span (ONS)on which the node is capable of directly detecting one or more eventsand thereby of originating the span's status, and in the non-originatingmode, a node is considered a non-originating node (NON) with respect toany other span (NONS) in the network, not associated with thenon-originating node, and wherein the step of circulating the FUmessages comprises performing the following actions at each network nodeand at each clock: for one or more spans (ONS) associated with said node(ON), originating respective status information; for spans (NONS) notassociated with said node (NON), forming respective status informationbased on the previous status information and status information in oneor more input FU messages available in said node; combining the statusinformation originated for ONS and the status information formed forNONS and forwarding it to all adjacent nodes as an output FU message ofsaid node.
 9. The method according to claim 8, wherein the step oforiginating status information for the ONS spans comprises: defining astate machine (SM, 20) of an originating node, assigned to a particularspan (ONS) associated with its originating node (ON), the state machinecomprising more than two states and providing transitions from state tostate; said states and transitions depending on presence or absence ofsaid one or more events on a particular span (ONS), wherein the statesof the state machine (SM) respectively correspond to possible statusesof a particular span (ONS); providing at each node of the network one ormore said state machines (SM) respectively assigned to one or more spans(ONS) associated to said node, thereby ensuring that status of eachparticular span of the network is originated and permanently updated atits associated ON.
 10. The method according to claim 8, wherein the stepof forming the status information for the NONS spans comprises: storing,at each node of the network, a previous output FU message of the nodereflecting the previous status information on all spans of the networkknown to the node; applying, at each node of the network, status rulesfor forming status information for the NONS spans based on therespective previous status information and respective status informationin the one or more input FU messages.
 11. The method according to claim10, wherein the status rules comprise a table of priorities identicalfor all nodes throughout the network.
 12. The method according to claim11, wherein the priorities are arranged by taking into account: theprevious status of a particular NONS transmitted by said node in theprevious output FU message, status(es) of the particular span NONSreceived at the node in one or more input FU messages, direction oftransitions of the state machine SM.
 13. The method according to claim12, wherein the priorities are stated as follows: the highest priorityis assigned to a status of the particular NONS span corresponding tosuch a state of the state machine SM which is positioned one transitionforward from the state of said machine corresponding to the previousstatus of the NONS span, the priority lower than but next to the highestis assigned to the previous status of said NONS span, i.e. to itsunchanged status, the lowest priority is assigned to a status of theparticular NONS span corresponding to such a state of the state machineSM, which is positioned one transition backward from the state of thestate machine SM corresponding to the previous status of the NONS span.14. The method according to claim 13, wherein the step of forming thestatus information for the NONS spans further comprises the followingsub-steps: if the status of a particular NONS span is equal in all inputFU messages which can be received at the node, accepting said equalstatus for the output FU message regardless a previous status of saidNONS; when receiving input FU messages carrying different statuses withrespect to one and the same NONS, selecting for the output FU messagesuch a status of said span which has the highest priority among thosereceived at said NON including the previous status of said NONS.
 15. Asystem for implementing the method according to claim
 1. 16. A systemfor dissemination of information about faults in spans that interconnectnodes in a telecommunication mesh network, the system comprises a faultupdating (FU) channel for circulating input and output FU messagesbetween nodes of the network, the FU messages carrying statusinformation on all network spans; the FU channel being arranged byproviding each node in the mesh network with means for receiving theinput FU messages from its adjacent nodes and means for transmitting theoutput FU messages to its adjacent nodes; each node being provided witha memory and a processing means for updating status information of allnetwork spans in its output FU messages.
 17. The system according toclaim 16, wherein each of the nodes is provided with means for detectingevents on one or more network spans associated with said node and astate machine SM for originating, based on said events, statusinformation concerning said spans (ONS); and also provided with statusrules for updating status information concerning network spans notassociated with said node (NONS).
 18. The system according to claim 17,wherein the memory of a node being utilized for storing a previousoutput FU message, the events, the SM, the input FU messages and thestatus rules SR; the processing means being operative to perform the SMtransitions to obtain status information of the ONS spans, capable ofapplying the status rules SR to form status information of the NONSspans, and adapted to combine the obtained status information of allnetwork spans in an output FU message of the node.